Consider refactor of query handler base to include JSON/Protobuf parsing explicitly

[ifadams]

Is your feature request related to a problem? Please describe.
Currently there is still some copy-paste work for doing the initial parse and validation of the incoming JSON, if possible we should try to generalize this to be inherited in the derived classes for better code re-use

Describe the solution you'd like
Assuming the current parsing and validation is general enough we should at a minimum move the JSON parsing and marshalling to a common base method in QueryHandler base, and will require some minor updates to derived classes (PMGD, Example Handler, and ongoing development of Neo4J handler)

Additional context
There's been discussion of generalizing around RS-Commands however, different handlers may require different signatures that are difficult to template. I will look into this, but this refactor will probably be minimal and focused around getting the JSON in order for general use in the derived classes.